1. Field of the Invention
The present invention relates to an optical line terminal (OLT), and more particularly to an OLT connected to a plurality of optical network units (ONUs), the OLT and the ONUs forming a passive optical network (PON). The present invention also relates to a method of optoelectrically converting a received signal.
2. Description of the Background Art
A telecommunications network connecting a station of a telecommunications carrier to subscriber premises equipment is defined as an access network. With a recent increase in communication traffic, in the field of access networks, optical access networks are now prevailing which accomplish transmission of an enormous volume of information by means of optical communication.
The optical access networks are exemplified by a PON. The PON includes an OLT installed in a carrier station, a plurality of ONUs installed in respective subscriber premises, and an optical splitter. The OLT is interconnected to the ONUs via the optical splitter by optical fibers.
The interconnection of the OLTs to the optical splitter is implemented by a single-core optical fiber. The single-core optical fiber is shared in use by the plurality of ONUs. Optical splitters are inexpensive passive devices. Thus, the PON is highly economical and can be readily maintained. Therefore, PONs are rapidly prevailing.
The PON relies upon various kinds of multiplex technique. Typical multiplex techniques used in the PON may be exemplified by the time division multiplex (TDM) allocating subscribers to short sections of a time axis, or time slots, the wavelength division multiplex (WDM) allocating subscribers to different wavelengths, the code division multiplex (CDM) allocating subscribers different codes. Among those multiplex schemes, the TDM-PON, using the TDM, has recently become most predominant, as disclosed in “Technical Fundamentals Lecture [GE-PON Technology]”, NTT Technical Journal, Vol. 17, No. 8, pp. 71-74, August 2005.
In the PON, communication signals transmitted from the respective ONUs toward the OLT, i.e. upstream signals, are combined in the optical splitter and then transmitted to the OLT. A communication signal transmitted from the OLT and meant for the respective ONUs, i.e. downstream signal, is split by the optical splitter into signals, which are in turn transmitted to the respective ONUs.
The upstream signal includes an upstream data signal and an upstream control signal. The upstream data signal is defined as a signal transmitted from user apparatus connected to the ONU toward an upper-layer network, e.g. application layer, connected to the OLT. In detail, the upstream data signal carries information the user requests the OLT to transmit to the upper-layer network. The upstream control signal is defined as a signal transmitted from the controller in the ONU toward the controller in the OLT for use in controlling the PON.
The downstream signal includes a downstream data signal and a downstream control signal. The downstream data signal is defined as a signal transmitted from the upper-layer network to the user apparatus. In detail, the downstream data signal carries information requested to be received by the user. The downstream control signal is defined as a signal transmitted from the controller in the OLT toward the controller in the ONU for use in controlling the PON.
The TDM-PON uses a time division multiple access (TDMA). The TDMA is a technique in which the OLT manages the ONUs to control the transmission timing therebetween so that an upstream signal from an ONU does not collide with an upstream signal from another ONU.
In the TDM-PON, the time axis is divided into a plurality of short slots. The short slots are allocated to the respective ONUs as bands, each of which different one ONU may use to transmit an upstream signal toward the OLT. Which transmission timing and which bandwidth each of the ONUs is to use in transmitting an upstream signal are instructed by the OLT to the ONU. The allocation of bands to the ONUs by the OLT as described above may be referred to as band allocation. In the context, the “bandwidth”maybe represented with respect to time, for example, seconds.
As disclosed in U.S. patent application publication No. US 2007/0122151 A1 to Watanabe, known band allocation procedures are exemplified by a dynamic bandwidth allocation (DBA). In the DBA, the OLT receives requests for transmission band from the ONUs and dynamically allocates communication bands on the basis of the requests.
In the above conventional band allocation procedure, after bands requested from all of the ONUs are allocated, if allocable bands still remain, they are allocated to the respective ONUs as surplus bands. In this case, the OLT itself is surely aware of receiving upstream signals in the periods corresponding to the requested bands, but not whether to receive an upstream signal in the periods corresponding to the surplus bands. Therefore, the OLT has to always keep rendering its receiver for upstream signal in the standby state also in the periods corresponding to the surplus bands. Unless the receiver receives an upstream signal in the corresponding periods, it wastes standby electricity.